The present invention relates to a power cable being electrically insulated by means of a synthetic material and being protected against the ingress of moisture.
High-power cables, particularly of the variety used for transmitting high voltages, are usually constructed in a multilayer configuration. The conductive core is covered or enveloped by a relatively thick insulation being, in turn, covered in some fashion by a shield. The shield is, for example, constituted by wires, or the like, embedded in a sealer serving as a barrier against ingress moisture (see, for example, German patent application No. 25,14,891); or a tube is used as a shield, constituting also a barrier against moisture (U.S. Pat. No. 3,943,271 or German patent application No. 15,40,430). This barrier shield is constructed from longitudinally folded strip, being lap-soldered or welded.
The known high-power cables of any type and construction, as described, are disadvantaged by the fact that, upon loading the cable, its insulation undergoes a different (usually much larger) thermal expansion than the enveloping metal shield structure. Soon the welding or soldering seam will break. Such local rupture does not immediately affect the cable performance, but moisture will penetrate, migrate longitudinally, e.g., underneath the metal envelope, and the thermal expansion of the insulation is actually gradually increased by that moisture. Hence, the "working" of the expanding insulation against the metal envelope is reenforced so that sooner or later the entire cable is destroyed.
Communication cables have been constructed to include swelling agents who swell when moist and, hopefully, plug a ruptured envelope. One has tried to use this technique in power cable, but unsuccessfully so. In particular, the introduction of swellable powder and the relatively large quantities needed have proven troublesome.